Compositions based on cerium oxide, zirconium oxide and, optionally, another rare earth oxide, having a specific raised surface at 1100°c, method for the production and use thereof as a catalyst

ABSTRACT

The inventive composition, according to a first embodiment, consists essentially of a cerium oxide and a zirconium oxide in an atomic ratio Ce/Zr of at least 1. According to a second embodiment, said composition is based on cerium oxide, zirconium oxide with an atomic ratio Ce/Zr of at least 1 and at least one rare earth oxide other than cerium. After calcination at 1100 C., said composition has a specific surface of at least 9 m2/g in the first embodiment and at least 19 m2/g in the second embodiment. The inventive composition can be used as a catalyst especially for the treatment of waste gases from internal combustion engines.

The present invention relates to compositions based on cerium oxide,zirconium oxide and, optionally, another rare earth oxide, having a highspecific surface area at 1100° C., to their method of preparation and totheir use as a catalyst.

At the present time, catalysts called “multifunctional” catalysts areused for the treatment of exhaust gases from internal combustion engines(by automobile postcombustion catalysis). Multifunctional catalysts areunderstood to mean those capable of carrying out not only oxidation, inparticular of carbon monoxide and the hydrocarbons present in exhaustgases, but also reduction, in particular of nitrogen oxides also presentin these gases (“three-way” catalysts). Today, zirconium oxide andcerium oxide appear to be two particularly important and advantageousconstituents for this type of catalyst. To be effective, these catalystsmust have a high specific surface area even at high temperature.

There is a need for catalysts capable of being used at increasingly hightemperatures and, for this to be so, having a specific surface area thatis very stable.

The object of the invention is therefore the development of a catalyticcomposition that can meet this requirement.

For this purpose, and according to a first embodiment, the compositionof the invention essentially consists of a cerium oxide and a zirconiumoxide in a Ce/Zr atomic ratio of at least 1, and it is characterized inthat it has a specific surface area of at least 9 m²/g after calcinationat 1100° C. for 4 hours.

According to a second embodiment, the composition of the invention isbased on a cerium oxide, a zirconium oxide in a Ce/Zr atomic ratio of atleast 1 and at least one oxide of a rare earth other than cerium, and itis characterized in that it has a specific surface area of at least 19m²/g after calcination at 1100° C. for 4 hours.

The invention also relates to a method of preparing such compositions,which is characterized in that it comprises the following steps:

(a) a mixture comprising a cerium compound, a zirconium compound and, ifapplicable, a compound of the aforementioned rare earth is formed;

(b) said mixture is brought into contact with a basic compound, by meansof which a precipitate is obtained;

(c) said precipitate is heated in aqueous medium; then

(d) an additive, chosen from anionic surfactants, nonionic surfactants,polyethylene glycols, carboxylic acids and their salts, and surfactantsof the carboxymethylated fatty alcohol ethoxylate type is added to theprecipitate obtained in the previous step; and

(e) the precipitate thus obtained is calcined.

As was mentioned above, the compositions of the invention have highspecific surface areas even after calcination at the high temperature of1100° C.

Other features, details and advantages of the invention will become evenmore fully apparent on reading the following description, and fromspecific but nonlimiting examples intended to illustrate it.

As regards the rest of the description, the term “specific surface area”is understood to mean the BET specific surface area determined bynitrogen adsorption in accordance with the ASTM D 3663-78 standardestablished on the basis of the Brunauer—Emmett—Teller method describedin the periodical The Journal of the American Chemical Society, 60, 309(1938).

Furthermore, the calcinations after which the surface areas are givenare calcinations in air.

The term “rare earth” is understood to mean the elements of the groupconsisting of yttrium and the elements of the Periodic Table havingatomic numbers between 57 and 71 inclusive.

Unless otherwise indicated, the contents are given as oxides. The ceriumoxide is in the form of ceric oxide.

It should be pointed out that, for the rest of the description, unlessotherwise indicated, the limit values are included in the ranges ofvalues given.

The compositions of the invention are in two embodiments that differ bythe nature of their constituents. In the first embodiment, thesecompositions essentially consist of cerium oxide and zirconium oxide. Bythis it should be understood that the composition does not containanother oxide of another element, which may be a stabilizer for thesurface area of said composition in the form of a rare earth other thancerium.

In the case of the first embodiment, the amount of cerium oxide in thecomposition is such that the Ce/Zr atomic ratio is at least 1, whichcorresponds to a proportion by weight of cerium oxide relative to theoverall composition of at least 58%. This proportion may be moreparticularly between 58% and about 80% and even more particularlybetween 58% and about 70%.

As indicated above, the compositions according to this first embodimentafter calcination at 1100° C. for 4 hours have a surface area that is atleast 9 m²/g. This surface area may more particularly be at least 10m²/g and even more particularly at least 15 m²/g. Moreover, aftercalcination at 1200° C. for 4 hours, these same compositions may stillhave a specific surface area of at least 2 m²/g, preferably at least 3m²/g and even more particularly at least 4 m²/g.

These same compositions may also have a specific surface area of atleast 45 m²/g after calcination at 900° C. for 4 hours. Finally, theymay have a specific surface area of at least 20 m²/g, preferably atleast 25 m²/g, after calcination at 1000° C. for 4 hours.

In the case of the second embodiment of the invention, the compositionsare based on cerium oxide, zirconium oxide and at least one oxide of arare earth other than cerium. In this case, the compositions thereforecontain at least three oxides and, more particularly, four. The rareearth other than cerium may especially be chosen from yttrium,lanthanum, neodymium and praseodymium and combinations thereof. Thus,mention may more particularly be made as compositions according to thissecond embodiment of those based on cerium oxide, zirconium oxide andlanthanum oxide, of those based on cerium oxide, zirconium oxide,lanthanum oxide and neodymium oxide and of those based on cerium oxide,zirconium oxide, lanthanum oxide and praseodymium oxide.

Still within the case of this second embodiment, the content of oxide ofthe rare earth other than cerium is generally at most 35%, especially atmost 20%, by weight relative to the overall composition. This contentmay more particularly be at most 15% and even more particularly at most10%. It is also usually at least 1% and more particularly at least 5%.

The respective cerium and zirconium contents in the compositions of thissecond type are such that the Ce/Zr atomic ratio is at least 1, whichcorresponds, as indicated above, to a proportion by weight of ceriumoxide relative to the overall composition of at least 58%. Thisproportion may here be more particularly between 58% and about 90% andeven more particularly between 58% and about 70%.

The compositions of the second embodiment have a specific surface areaof at least 19 m²/g after calcination at 1100° C. for 4 hours. Thissurface area may be more particularly at least 21 m²/g.

Furthermore, after calcination at 1200° C. for 4 hours, these samecompositions may still have a specific surface area of at least 3 m²/g,preferably at least 4 m²/g and even more particularly at most 6 m²/g.

These same compositions may also have a specific surface area of atleast 60 m²/g, more particularly at least 65 m²/g, after calcination at900° C. for 4 hours. Finally, they may have a specific surface area ofat least 35 m²/g, preferably at least 40 m²/g, after calcination at1000° C. for 4 hours.

According to another feature, the compositions may advantageously be inthe form of a solid solution. The X-ray diffraction spectra of thesecompositions reveal in this case, within the latter, the existence of apure or homogeneous single phase. This phase in fact corresponds to acrystalline structure of the fluorite type, just like the crystallizedceric oxide CeO₂, the lattice parameters of which are somewhat shiftedrelative to a pure ceric oxide, thus reflecting the incorporation of thezirconium and, if applicable, the other rare earth into the crystallattice of cerium oxide, and therefore the formation of a true solidsolution. In the case of the compositions of the first embodiment, thispure phase is preserved up to a temperature of 1000° C. In the case ofthe compositions of the second embodiment, it is preserved up to atemperature of 1100° C.

The method of preparing the compositions of the invention will now bedescribed.

This method is characterized in that it comprises the following steps:

(a) a mixture comprising a cerium compound, a zirconium compound and, ifapplicable, a compound of the aforementioned rare earth is formed;

(b) said mixture is brought into contact with a basic compound, by meansof which a precipitate is obtained;

(c) said precipitate is heated in aqueous medium; then

(d) an additive, chosen from anionic surfactants, nonionic surfactants,polyethylene glycols glycols, carboxylic acids and their salts, andsurfactants of the carboxymethylated fatty alcohol ethoxylate type isadded to the precipitate obtained in the previous step; and

(e) the precipitate thus obtained is calcined.

The first step (a) of the method therefore consists in preparing amixture in a liquid medium of a zirconium compound, a cerium compoundand optionally at least one compound of the additional aforementionedrare earth.

The mixing is generally carried out in a liquid medium, which ispreferably water.

The compounds are preferably soluble compounds. These may especially bezirconium, cerium and lanthanide salts. These compounds may be chosenfrom nitrates, sulfates, acetates, chlorides and ceric ammoniumnitrates.

As examples, mention may thus be made of zirconium sulfate, zirconylnitrate or zirconyl chloride. Most generally, zirconyl nitrate is used.Mention may also be especially be made of cerium (IV) salts such as, forexample, nitrates or ceric ammonium nitrates, which are particularlysuitable here. Ceric nitrate is preferably used. It is advantageous touse salts with a purity of at least 99.5% and more particularly at least99.9%. An aqueous ceric nitrate solution may for example be obtained bythe reaction of nitric acid on a hydrated ceric oxide preparedconventionally by reacting a solution of a cerous salt, for examplecerous nitrate, with an ammonia solution in the presence of hydrogenperoxide. It is also possible preferably to use a ceric nitrate solutionobtained by the method of electrolytic oxidation of a cerous nitratesolution, as described in the document FR-A-2 570 087, which constituteshere an advantageous raw material.

It should be noted here that the aqueous solutions of cerium salts andzirconyl salts may have a certain initial free acidity, which can beadjusted by the addition of a base or an acid. However, it is equallypossible to employ an initial solution of cerium and zirconium saltshaving actually a certain free acidity as mentioned above and solutionsthat will have been neutralized beforehand to a greater or lesserextent. This neutralization may be carried out by the addition of abasic compound to the aforementioned mixture so as to limit thisacidity. This basic compound may for example be an ammonia solution or asolution of alkali metal (sodium, potassium, etc.) hydroxides, butpreferably an ammonia solution.

Finally, it should be noted that, when the starting mixture containscerium in the (III) form, it is preferable to employ, during the method,an oxidizing agent, for example hydrogen peroxide. This oxidizing agentmay be used by being added to the reaction mixture during step (a) orduring step (b), especially at the end of the latter step.

It is also possible to use a sol as starting compound of zirconium orcerium. The term “sol” denotes any system consisting of fine solidparticles of colloidal dimensions, that is to say dimensions of betweenabout 1 nm and about 500 nm, based on a zirconium or cerium compound,this compound generally being a zirconium or cerium oxide and/orhydrated oxide, in suspension in an aqueous liquid phase, said particlesfurthermore optionally being able to contain residual amounts of bondedor adsorbed ions, such as for example nitrate, acetate, chloride orammonium ions. It should be noted that, in such a sol, the zirconium orcerium may be either completely in the form of colloids, orsimultaneously in the form of ions and in the form of colloids.

It does not matter whether the mixture is obtained from compoundsinitially in the solid state, which will subsequently be introduced intoan aqueous stock for example, or directly from solutions of thesecompounds, said solutions then being mixed in any order.

In the second step (b) of the method, said mixture is brought intocontact with a basic compound. As base or basic compound, it is possibleto use products of the hydroxide type. Mention may be made of alkalimetal or alkaline-earth metal hydroxides. It is also possible to usesecondary, tertiary or quaternary amines. However, amines and aqueousammonia may be preferred in so far as they reduce the risks ofcontamination by alkali metal or alkaline-earth metal cations. Mentionmay also be made of urea.

The way in which the mixture and the solution are brought into contactwith each other, that is to say the order of introduction thereof, isnot critical. However, this contacting may be carried out by introducingthe mixture into the solution of the basic compound. This variant ispreferable in order to obtain compositions in the form of solidsolutions.

The contacting or the reaction between the mixture and the solution,especially the addition of the mixture into the solution of the basiccompound, may be carried out in a single step, gradually orcontinuously, and it is preferably performed with stirring. It ispreferably carried out at room temperature.

The next step (c) of the method is the step of heating the precipitatein aqueous medium.

This heating may be carried out directly on the reaction mixtureobtained after reaction with the basic compound or on a suspensionobtained after separating the precipitate from the reaction mixture,optionally washing it and putting it back into water. The temperature atwhich the medium is heated is at least 100° C. and even more preferablyat least 130° C. The heating operating may be carried out by introducingthe liquid medium into a sealed chamber (a closed reactor of theautoclave type). Under the temperature conditions given above, and inaqueous medium, it may thus be specified, by way of illustration, thatthe pressure in the closed reactor may vary between a value greater than1 bar (10⁵ Pa) and 165 bar (1.65×10⁷ Pa), preferably between 5 bar(5×10⁵. Pa) and 165 bar (1.65×10⁷ Pa). The heating may also be carriedout in an open reactor for temperatures close to 100° C.

The heating may be carried out either in air or in an inert gasatmosphere, preferably in nitrogen.

The duration of the heating may vary widely, for example between 1 and48 hours, preferably between 2 and 24 hours. Likewise, the rate at whichthe temperature rises is not critical—it is thus possible to reach thefixed reaction temperature by heating the medium for example between 30minutes and 4 hours, these values being given merely by way ofindication.

The heated medium generally has a pH of at least 5. Preferably, this pHis basic, that is to say it is greater than 7 and more particularly atleast 8.

It is possible to carry out several heating operations. Thus, theprecipitate obtained after the heating step and optionally a washingoperation may be resuspended in water and then another heating operationmay be carried out on the medium thus obtained. This other heatingoperation is carried out under the same conditions as those describedfor the first one.

The next step (d) of the method consists in adding, to the precipitateobtained after the preceding step, an additive that is chosen fromanionic surfactants, nonionic surfactants, polyethylene glycols, andcarboxylic acids and their salts.

As regards this additive, reference may be made to the teaching ofApplication WO-98/45212 and the surfactants described in that documentmay be used.

Mention may be made, as surfactants of the anionic type, ofethoxycarboxylates, ethoxylated fatty acids, sarcosinates, phosphateesters, sulfates such as alcohol sulfates, alcohol ether sulfates andsulfated alkanolamide ethoxylates, sulfonates such as sulfosuccinates,alkylbenzenesulfonates or alkylnaphthalenesulfonates.

As nonionic surfactants, mention may be made of acetylenic surfactants,alcohol ethoxylates, alkanolamides, amine oxides, ethoxylatedalkanolamides, long-chain ethoxylated amines, ethylene oxide/propyleneoxide copolymers, sorbitan derivatives, ethylene glycol, propyleneglycol, glycerol, polyglyceryl esters and their ethoxylated derivatives,alkylamines, alkylimidazolines, ethoxylated oils and alkylphenolethoxylates. Mention may be made in particular of the products soldunder the brand names IGEPAL®, DOWANOL®, RHODAMOX® and ALKAMIDE®.

As regards carboxylic acids, it is possible to use in particularaliphatic monocarboxylic or dicarboxylic acids and, among these,saturated acids may more particularly be used. Use may also be made offatty acids and more particularly of saturated fatty acids. Thus,mention may especially be made of formic, acetic, propionic, butyric,isobutyric, valeric, caproic, caprylic, capric, lauric, myristic andpalmitic acids. As dicarboxylic acids, mention may be made of oxalic,malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic andsebacic acids.

The salts of carboxylic acids may also be used, especially ammoniacalsalts.

As an example, mention may more particularly be made of lauric acid andammonium laurate.

Finally, it is possible to use a surfactant chosen from those of thecarboxymethylated fatty alcohol ethoxylate type.

The term “product of the carboxymethylated fatty alcohol ethoxylatetype” is understood to mean products composed of ethoxylated orpropoxylated fatty alcohols having a CH₂—COOH group at the chain end.

These products may correspond to the formula:R₁—O— (CR₂R₃—CR₄R₅—O)_(n)—CH₂—COOH,in which R₁ denotes a saturated or unsaturated carbon chain, the lengthof which is generally at most 22 carbon atoms, preferably at least 12carbon atoms; R₂, R₃, R₄ and R₅ may be identical and represent hydrogenor else R₂ may represent a CH₃ group and R₃, R₄ and R₅ representhydrogen; n is a nonzero integer that can range up to 50 and moreparticularly is between 5 and 15, these values being inclusive. Itshould be noted that a surfactant may consist of a mixture of productsof the above formula in which R₁ may be saturated and unsaturatedrespectively or else products comprising both —CH₂—CH₂—O— and—C(CH₃)—CH₂—O— groups.

The addition of the surfactant may be carried out in two ways. It may beadded directly to the suspension of precipitate obtained from thepreceding heating step (c).

It may also be added to the solid precipitate after this has beenseparated by any known means from the medium in which the heating tookplace.

The amount of surfactant used, expressed as a percentage by weight ofadditive relative to the weight of the composition calculated as oxide,is generally between 5% and 100%, more particularly between 15% and 60%.

According to an alternative way of implementing the method, it ispossible to carry out a moderate-energy milling operation on theprecipitate in suspension, by subjecting this suspension to a shearingaction, for example using a colloid mill or a turbine agitator.

In a final step of the method according to the invention, the recoveredprecipitate is then calcined. This calcination allows the crystallinityof the product formed to be increased, and it may also be adjustedand/or chosen depending on the subsequent use temperature reserved forthe composition according to the invention, taking into account the factthat the specific surface area of the product is lower the higher thecalcination temperature employed. Such a calcination is generallycarried out in air, but a calcination carried out for example in aninert gas or in a controlled (oxidizing or reducing) atmosphere is ofcourse not excluded.

In practice, the calcination temperature is generally limited to a rangeof values between 300 and 1000° C.

The compositions of the invention, as described above or as obtained inthe method studied above, are in the form of powders, but they mayoptionally be formed into granules, beads, cylinders or honeycombs ofvarying dimensions.

The compositions of the invention may be used as catalysts or ascatalyst supports. Thus, the invention also relates to catalytic systemscomprising the compositions of the invention. For such systems, thesecompositions may be applied to any support normally used in thecatalysis field, that is to say, in particular, thermally inertsupports. This support may be chosen from alumina, titanium oxide,cerium oxide, zirconium oxide, silica, spinels, zeolites, silicates,crystalline silicon aluminum phosphates and crystalline aluminumphosphates.

The compositions may also be used in catalytic systems including a washcoat having catalytic properties and based on these compositions, on asubstrate for example of the metal or ceramic monolith type. The washcoat may itself include a support of the type of those mentioned above.This wash coat is obtained by mixing the composition with the support soas to form a suspension that may then be deposited on the substrate.

These catalytic systems, and more particularly the compositions of theinvention, may have very numerous applications. They are thusparticularly well suited to, and therefore usable in, the catalysis ofvarious reactions such as, for example, dehydration, hydrosulfurization,hydrodenitrification, desulfurization, hydrodesulfurization,dehydrohalogenation, reforming, steam reforming, cracking,hydrocracking, hydrogenation, dehydrogenation, isomerization,dismutation, oxychlorination and dehydrocyclization of hydrocarbons orother organic compounds, oxidation and/or reduction reactions, the Clausreaction, the treatment of internal combustion exhaust gases,demetalization, methanation, shift conversion, catalytic oxidation ofsoot emitted by internal combustion engines, such as diesel or petrolengines operating in lean mode. Finally, the catalytic systems and thecompositions of the invention may be used as NOx traps.

In the case of these uses in catalysis, the compositions of theinvention are employed in combination with precious metals, saidcompositions thus acting as support for these metals. The nature ofthese metals and the techniques of incorporating them into supportcompositions are well known to those skilled in the art. For example,the metals may be platinum, rhodium, palladium or iridium, and they mayespecially be incorporated into the compositions by impregnation.

Among the uses mentioned, the treatment of internal combustion engineexhaust gases (by automobile postcombustion catalysis) constitutes oneparticularly advantageous application. Consequently, the invention alsorelates to a method of treating the exhaust gases of internal combustionengines, which is characterized in that a catalytic system as describedabove or a composition according to the invention and as described aboveis used as catalyst.

Examples will now be given.

EXAMPLE 1

This example relates to the preparation of a composition based on ceriumoxide and zirconium oxide in respective proportions by weight of 58/42.

Introduced into a stirred beaker were 525 ml of zirconium nitrate (80g/l) and 230 ml of ceric nitrate (Ce⁴⁺=236.5 g/l; Ce³⁺=15.5 g/l; andfree acid=0.7N) Distilled water was then added so as to obtain 1 literof a solution of nitrates.

Introduced into a stirred round-bottomed reactor were 253 ml of anaqueous ammonia solution and distilled water was then added so as toobtain a total volume of 1 liter.

The solution of nitrates was introduced over 1 hour into the reactorwith constant stirring.

The solution obtained was placed in a stainless steel autoclave fittedwith a stirrer. The temperature of the medium was raised to 150° C. over2 hours with stirring.

The suspension this obtained was then filtered on a Buchner funnel. Aprecipitate containing 23.4% oxide by weight was recovered.

100 g of this precipitate were taken.

In parallel, an ammonium laurate gel was prepared under the followingconditions: 250 g of lauric acid were introduced into 135 ml of aqueousammonia (12 mol/l) and 500 ml of distilled water, and then the mixturewas homogenized using a spatula.

28 g of this gel were added to 100 g of the precipitate and thecombination was then mixed until a uniform paste was obtained.

The product obtained was then heated to 650° C. for 2 hours in stages.

Indicated below are the surface areas obtained after subsequentcalcinations at various temperatures:

4 h at 900° C.: 49 m²/g;

4 h at 1000° C.: 31 m²/g;

4 h at 1100° C.: 15 m²/g;

10 h at 1200° C.: 4 m²/g.

EXAMPLE 2

This example relates to the preparation of a composition based oncerium, zirconium, lanthanum and praseodymium oxides in the respectiveproportions by weight of 60%, 30%, 3% and 7%.

Introduced into a stirred beaker were 375 ml of zirconium nitrate (80g/l), 121 ml of cerium nitrate in the oxidation state III (496 g/l), 6.6ml of lanthanum nitrate (454 g/l) and 14 ml of praseodymium nitrate (500g/l). Distilled water was then added so as to obtain 1 liter of asolution of nitrates.

Introduced into a stirred round-bottomed reactor were 200 ml of anaqueous ammonia solution (12 mol/l), 302 ml of hydrogen peroxide (110volumes) and distilled water was then added so as to obtain a totalvolume of 1 liter.

The procedure then continued as in Example 1, and the suspensionobtained after the autoclave treatment was then filtered on a Buchnerfunnel. A precipitate containing 30.5% oxide by weight was recovered.

100 g of this precipitate were taken and 36.5 g of an ammonium laurategel was added to them, said gel being prepared as in Example 1, until auniform paste was obtained.

The product obtained was then heated to 650° C. for 2 hours in stages.

Indicated below are the surface areas obtained after subsequentcalcinations at various temperatures:

4 h at 900° C.: 65 m²/g;

4 h at 1000° C.: 42 m²/g;

4 h at 1100° C.: 23 m²/g;

10 h at 1200° C.: 4 m²/g.

1-13. (canceled)
 14. A composition essentially consisting of a ceriumoxide and a zirconium oxide in a Ce/Zr atomic ratio of at least 1, saidcomposition having a specific surface area of at least 9 m²/g aftercalcination at 1100° C. for 4 hours.
 15. The composition as claimed inclaim 14, having a specific surface area of at least 15 m²/g aftercalcination at 1100° C. for 4 hours.
 16. The composition as claimed inclaim 14, having a specific surface area of at least 2 m²/g, optionallyat least 4 m²/g, after calcination at 1200° C. for 4 hours.
 17. Acomposition based on a cerium oxide, a zirconium oxide in a Ce/Zr atomicratio of at least 1 and at least one oxide of a rare earth other thancerium, having a specific surface area of at least 19 m²/g aftercalcination at 1100° C. for 4 hours.
 18. The composition as claimed inclaim 16, wherein the specific surface area is of at least 3 m²/g,optionally at least 6 m²/g.
 19. The composition as claimed in claim 17,further comprising at least one oxide of a rare earth other than cerium,selected from the group consisting of lanthanum, neodymium andpraseodymium.
 20. The composition as claimed in claim 19, wherein theoxide of a rare earth other than cerium is present in the composition inan amount of at least 20% by weight.
 21. A method of preparing acomposition as claimed in claim 14, comprising the steps of: (a) forminga mixture comprising a cerium compound, a zirconium compound and,optionally, a compound of a rare earth other than cerium, selected fromthe group consisting of lanthanum, neodymium and praseodymium; (b)bringing said mixture into contact with a basic compound, to obtain aprecipitate; (c) heating said precipitate in an aqueous medium; (d)adding an additive, selected from the group consisting of anionicsurfactants, nonionic surfactants, polyethylene glycols, carboxylicacids, the salts of carboxylic acids, and carboxymethylated fattyalcohol ethoxylate surfactants, to the precipitate obtained in theprevious step; and, then (e) calcining the precipitate obtained in step(d).
 22. The method as claimed in claim 21, wherein, the zirconiumcompound, cerium compound and compound of the rare earth, is selectedfrom the group consisting of nitrates, acetates, chlorides and cericammonium nitrates.
 23. The method as claimed in claim 21, wherein, inthe mixture of step (a), a the cerium compound is in the Ce(III) formand an oxidizing agent is added during step (a) or during step (b),optionally at the end of the latter step.
 24. The method as claimed inclaim 21, wherein the precipitate is heated in step (c) to a temperatureof at least 100° C.
 25. A catalytic system, comprising a composition asclaimed in claim
 14. 26. A method of automobile postcombustion catalysisof exhaust gases of an internal combustion engine, said methodcomprising the step of treating said exhaust gases with a catalyticsystem as claimed in claim 25.